1. Field of the Invention
The invention relates to microwave systems generally, and more particularly to an RF feed operative between fixed transmit/receive apparatus and a rotating antenna array, or the like.
2. Description of the Prior Art
The general problem of providing a rotating RF transmission line joint between a rotating antenna system and fixed transmitter, receiver and signal processing apparatus is nearly as old as the radar itself. Various forms of such rotating joints or couplings have been developed and are known in this art. A few of these include the rotating circular waveguide joint, the rotating coaxial coupling, and various hybrid arrangements in which there are one or more transitions from one transmission line medium to another.
The unique problem in shipboard radar arises when, for reasons intended to minimizing antenna blockage from ship superstructure, a rotating antenna is essentially mounted at the top of a mast. In such cases, it is particularly useful to provide some form of around-the-mast coupler, a part of which rotates with the antenna array and a part of which remains fixed for connection to the fixed apparatus, typically a transmitter, receiver and other signal processing equipment. One form of such a coupler is described in U.S. Patent Application Ser. No. 040,325 filed May 18, 1979, and is entitled "Around-The-Mast Rotary Coupler." That patent application is assigned to the assignee of the present application. In it, two matching, annular, cellular rings rotate with respect to each other. The cells of each of these rings are actually waveguide sections and energy transfer is effected during rotation.
One inherent problem associated with the aforementioned rotary coupler is this same fact, namely that the individual cells of the annular rings are in effect short sections of waveguides, and are therefore subject to the low frequency cut-off characteristic of waveguide. This means that, for relatively low microwave frequencies, the cross-sectional dimensions of these waveguide cells becomes relatively large. The result in size, weight and cost factors can be disadvantageous.
Of further interest in the prior art is U.S. Patent Application Ser. No. 77,850, filed Sept. 21, 1979 and entitled "Loop Coupler Commutating Feed." In that disclosure, the concept of fixed and rotating loops coupling to each other is presented and could be adapted to the rotary coupler use, however its diameter is relatively large since the loops are radially oriented. More importantly, however, it is not adapted to the around-the-mast configuration, and is essentially useful as background hereto because of the basic, coupled, elongated loop concept which it discloses in common with the invention herein.
Also of interest as background is U.S. Patent Application Ser. No. 19,481, filed Mar. 12, 1979 and entitled "Large Scale Low-Loss Combiner and Divider." That device, which does not involve moving parts is not a coupler per se, and is not directly applicable to the around-the-mast rotary coupler application, but like the aforementioned U.S. Patent Application Ser. No. 77,850 employs magnetically coupled elongated loops in independent input/output groups.
An annular RF coupler of the form to which the present invention is particularly applicable is described in a copending U.S. Patent Application entitled "Around-The-Mast Rotary Annular Antenna Feed Coupler," George A. Hockham and Ronald I. Wolfson inventors and identified by the assignee as Hockham-Wolfson 15-5.
All four of the aforementioned background patent applications are assigned to the same assignee as is the present invention.
In consideration of the background art and the particular problem to be solved, the manner in which the invention advances the state of this art will be understood as this description proceeds.